Moisture curable organosiloxane compositions find use in various applications, for example as sealant compositions that can be applied to a joint between elements and cured to provide an elastomeric seal between them. These compositions cure at room temperature and are particularly attractive for sealing, for example highway joints, joints in articles such as vehicle headlights and joints in buildings and in glazing applications, because no special heating or other cure conditions are generally required to produce a seal of desired quality.
Many moisture curable organosiloxane compositions have been proposed and are generally formed from an at least one substantially linear polyorganosiloxane containing at least two silanol groups, a crosslinker capable of reaction with the polyorganosiloxane to yield a crosslinked network, and catalyst materials. These compositions cure by a condensation reaction promoted by moisture.
The crosslinker in moisture curable organosiloxane compositions is generally selected from polyfunctional silanes that readily hydrolyze. Commonly employed crosslinkers are triacetoxy silanes, trialkoxy silanes, triamino silanes and trioximo silanes. It is believed that the condensation reaction proceeds via a capping of the polyorganosiloxane with, for example, dialkoxyalkylsilyl groups followed by interaction of the alkoxy groups of the end caps and or silanol groups to yield a crosslinked structure.
While some curing of the composition during manufacture and storage is acceptable, it is important that this curing does not proceed too far prior to application at its intended work site, at which it is intended to cure under influence of atmospheric moisture. Thus the exposure of the composition to moisture should be kept to a uniform, acceptably low extent from batch to batch during manufacture and storage, otherwise the composition cures to an extent that renders it impractical for its intended purpose.
Moisture curable compositions based on organosilicon compounds generally contain finely divided fillers. The fillers generally used are those that strengthen the cured material, reduce the cost of the product or otherwise confer a desired combination of properties.
Typical fillers include but are not limited to high surface area silicas, ground quartz, iron oxide, zinc oxide, carbon black, calcium carbonate and diatomaceous earth. Moisture curable organosiloxane compositions can be manufactured using a batch or continuous process during which the filler and polyorganosiloxane are mixed together, the crosslinker and catalyst are added to the mixture and the resultant composition is then packaged in containers such as cartridges, pails, or drums, which are then sealed in an airtight manner to prevent ingress of moisture.
Silicone pressure-sensitive adhesives (hereinafter also referred to as PSAs) typically contain at least two primary components, namely a linear siloxane polymer and a tackifier resin consisting essentially of triorganosiloxane units (i.e., R3SiO1/2 units. in which R denotes a monovalent organic group) and silicate units (i.e., SiO4/2 units). In addition to the above two ingredients, some silicone PSA compositions contain some crosslinking means (e.g., peroxide or hydrosilylation cure systems) in order to optimize various properties of the final adhesive product. In view of the high viscosity imparted by the polymer component, these PSA compositions are typically dispersed in an organic solvent for ease of application. Some of these PSAs contain reactive groups, which allow the compositions to be cured by exposure to moisture. When the proportions of the above described resin and polymer and other parameters are adjusted similar combinations can be formulated into coating compositions. Under certain other conditions without the use of solvents, hot melt PSAs can be obtained.